In recent years, zoom lenses have become popular lenses in imaging devices such as photographic and video cameras. Presently, high-zoom-ratio zoom lenses (i.e., zoom lenses having a zoom ratio greater than three) represent the state of the art and are standard on most advanced lens-shutter cameras and video equipment. The zoom lens for a lens-shutter type camera is most often integrated with the camera body and not readily detachable. Thus, it is useful for such cameras to be compact and lightweight. Not surprisingly, this has resulted in a demand for increasingly compact and lightweight zoom lenses having a high zoom ratio.
However, if the maximum telephoto state focal length is increased to increase the zoom ratio, the overall lens length and lens diameter increases, making the lens non-compact. On the other hand, if the maximum wide-angle state focal length is decreased to increase the zoom ratio, the amount of light at the margins of the field becomes attenuated, i.e., the image irradiance at the edges of the field is diminished due to the "cosine-to-the-fourth" dependence of image irradiance with field angle. Consequently, the marginal ray heights need to be increased to provide a sufficient amount of light at the margins of the field. This requires the lens diameter to be increased, resulting in a non-compact design.
Presently, the conventional zoom lens for a single-lens reflex (SLR) 35 mm camera having a mid-range focal length of about 50 mm is a retro-focus arrangement. An often-used retro-focus arrangement is a four-group construction-type comprising, from objectwise to imagewise, a positive lens group, a negative lens group, a positive lens group, and a positive lens group. Such a zoom lens has several advantages over other designs. For example, in a positive-negative-positive three-group construction-type, it is difficult to achieve a high zoom ratio while maintaining high-quality imaging performance. In fact, the more robust four-lens group construction-type is derived from the three-lens group construction-type by dividing the most imagewise positive lens group in the three-lens group construction-type into two separate and axially movable positive lens groups. This four-lens group construction-type allows the air space between these last two lens groups to be varied, providing for better control of aberrations while zooming.
Another advantage of the above four-lens group construction-type is that the second lens group can have strong negative refractive power, which is needed to keep the Petzval sum in check. Still another advantage is that the refractive power of the individual lens groups can be significantly strengthened to achieve a higher zoom ratio and greater compactness. This is possible because as the refractive power of the first lens group increases, the axial separation between the first and third lens groups can be reduced by increasing the negative refractive power of the second lens group.
Thus, efforts to develop a more compact, high-zoom-ratio zoom lens have been directed towards improving the positive-negative-positive-positive construction-type. In particular, efforts have been directed to shrinking the axial length in the maximum telephoto state, while decreasing the lens diameter in the maximum wide-angle state.
However, this approach has its limits and cannot be applied ad infinitum, mainly for two reasons. The first reason is that for positive-negative-positive-positive construction-type zoom lenses, the aperture stop is generally located objectwise and adjacent the third or fourth lens group. This reduces the ability of the lens elements in the second lens group to perform aberration correction because the ray heights do not vary significantly over the zooming range. Consequently, the number of lens elements needed to correct aberrations increases, which increase costs and prevents compactness from being achieved. Further, aspherical lens elements are usually required to obtain high-quality imaging, which can drastically increase cost. The second reason is that in the continued attempt to achieve compactness, the refractive power of the second lens group needs to be increased, as mentioned above. Eventually, however, this power starts to become disproportionate with respect to the other lens groups, making aberration control very difficult.